This invention relates to steels, and, more particularly, to a steel that achieves good structural properties with low alloying and production costs.
Low-alloy steels are iron-based metallic alloys, containing additional alloying elements in amounts of up to about 2 percent by weight, that are used in a wide variety of applications. Such steels typically have good mechanical and physical properties, generally low cost, and a high degree of versatility. Their properties can be varied over wide ranges by varying the alloying elements and processing of the steel to its final form.
The present invention deals with steels used in structural applications, such as beams, plates, bars, and the like. Such steels have medium levels of alloying elements that are, on the whole, relatively inexpensive. They are processed by casting and hot rolling, sometimes with accelerated cooling after rolling to improve the final mechanical properties. The properties of the final processed steel pieces depend upon their composition, processing, and final thickness. Thinner sections usually have properties superior to those of otherwise identical, but thicker, sections.
To improve the uniformity of such steels for their users, standards have been established for these and other types of steels by organizations such as the American Society for Testing and Materials (ASTM). In some examples of interest here, ASTM Specification A36 sets forth the chemical and physical requirements for a "plain carbon" steel having a minimum yield point of 36 thousand pounds per square inch (KSI). This steel is inexpensive, having no expensive alloying elements and being processed by casting and hot rolling. ASTM A529 establishes standards for a somewhat similar grade of steel, except that this steel achieves a minimum yield point of 42 KSI in sections of maximum thickness 1/2 inch. ASTM A572 defines standards for steels that achieve specified minimum yield strengths such as 42 KSI or 50 KSI in thicker sections, but at the cost of the use of more expensive alloying additions such as vanadium or niobium. (There are, of course, many other grades of steels with other sets of properties, but the three standards just discussed are of the most interest in relation to the present invention.) Many suppliers of steel products can supply any or all of these grades, but at varying costs depending upon the cost of the alloying elements and the processing.
These standards are used by structural designers to order steels that meet particular strength requirements, at minimum cost. If, for example, the designer requires I-beams with only a 36 KSI yield point steel, then the most inexpensive grades meeting ASTM A36 can be used. If a 42 KSI yield point is required in a thin section, an ASTM A529 grade steel might be ordered. If a 42 or 50 KSI yield point is required in a thicker section, a more expensive steel meeting ASTM A572 would be specified.
The various steel standards typically specify property levels that must be attained and maximum levels of alloying elements, but not minimum levels of alloying elements. A continuing effort by steelmakers is therefore to develop steels that meet the property requirements of the standards, but with reduced cost as a consequence of reduced levels of the more expensive alloying elements. In particular, it would be desirable to develop a steel that meets ASTM A572 Grade 50 or ASTM A529/A572 Grade 42 properties, but at lower costs than possible with the existing steels used for these grades. The present invention provides such steels, and their processing.